The present invention relates to a device for machining optical workpieces, in particular optical lenses, having a frame, which comprises two side walls, which support a horizontally displaceable gantry with at least one vertically displaceable tool spindle and the space between which takes the form of a machining area, in which a yoke comprising a workpiece holding means is provided, which yoke is mounted on the side walls by means of journals so as to be rotatable about a swivel axis.
Such devices are known from metal working using machine tools, in particular drilling and milling machines (EP 0 712 682 B1). These machines are constructed according to the xe2x80x9cgantryxe2x80x9d design, i.e. they have a gantry with a cross slide and a vertically disposed tool spindle, which may be displaced horizontally and vertically along two axes. One or more workpiece holders may be arranged in a swivellable yoke arranged beneath the tool spindle. The workpieces are either operated rotationally by means of a spinning table or are merely attached stationarily in the yoke.
It is common to all known machines in which a swivellable yoke is used for the swivelling motion always to be generated by a servo motor via appropriate gearing. In a known machine (DE 36 24 284 C2, FIG. 4), a reducing gear is used to swivel the yoke, a rotary transmission leadthrough with a drive fitted outside the yoke being additionally provided for rotary drive of the workpieces. However, this multi-component, mechanically complex gearing system inevitably exhibits backlash, which has a very disadvantageous effect on the repeatability of the angle to be set. However, angular adjustment precision is very important, in particular in the case of grinding spherical optical surfaces.
The object of the invention is to provide a device for machining optical workpieces which structurally follows the gantry design, but allows high-precision cutting machining even of large optical components with low mechanical complexity even in the event of high process forces and/or varying temperature influences.
According to the present invention, there is provided a device for machining optical workpieces, in particular optical lenses, having a frame, which comprises two side walls, which support a horizontally displaceable gantry with at least one vertically displaceable tool spindle, there being a space therebetween which takes the form of a machining area, in which a yoke comprising a workpiece holding means is provided, which yoke is mounted on the side walls by means of journals so as to be rotatable about a pivot axis; wherein, to swivel the yoke, a torque motor is provided which is attached to a side wall of the frame concentrically to the swivel axis and is connected actively with the associated journal of the yoke.
Taking as basis the above-stated prior art (EP 0 712 682 B1), claim 1 provides, while avoiding any gear arrangements, a simply constructed device in which no backlash may occur and which therefore allows the achievement of highly and reproducibly precise yoke swivelling movements and thus angular adjustment. The torque motor transmits the swivelling movements directly to the yoke.
If the yoke weight is compensated by counterweights, the torque motor has only to apply a very slight torque in order to bring the entire yoke, with the workpiece holding means/workpiece spindles and the workpieces, into the respective angular position. Only low motor power has therefore to be provided, which is favourable from the point of view of energy consumption and also does not cause excessive heating, which would be detrimental to adjusting and machining precision.
If the counterweights are arranged relative to the side walls of the frame, a narrow, high rigidity yoke is achieved, which in turn benefits machining precision.
The attachment, of a frictionally or interlockingly operating brake allows reliable locking of the yoke in any set angular position, whereby the range of possible geometries which may be produced by the device may be extended. A suitable, pneumatically or hydraulically actuated device may be provided to unlock the brake.
The devices may be such that the journal of the yoke actively connected with the torque motor is mounted in the corresponding side wall of the frame with a swivel bearing on each side of the torque motor and wherein the swivel bearing lying nearest the machining area takes the form of a fixed bearing.
This is an advantageous configuration of the swivel bearing arrangement for the yoke on the bearing journal connected actively with the torque motor. This ensures that, even in the event of heating of the yoke, no axial displacement of the yoke and thus of the workpiece holding means/workpiece spindles relative to the tool spindle may occur as a result of thermal expansion. Possible thermal expansion in the journal area is thus unable to spread into the machining area of the device, a fact which is conducive to the achievement of high machining precision. However, this thermosymmetrical construction of the device not only provides considerable advantages in the thermal respect but also with regard to the stability of the machine structure relative to the deformation tendencies caused by process forces. The construction according to the invention of the device also allows the area of the swivel bearings to be specifically temperature-controlled by water cooling, in particular in the area of the torque motor.
The swivelling movements performed by the yoke may be detected and controlled by a high-resolution rotary transducer. This direct detection of the angle of rotation allows high control quality and thus likewise serves in achieving the desired high machining precision.
A horizontal slide for two tool spindles is preferably provided on the horizontally displaceable gantry, which two spindles may be displaced independently of one another in the vertical direction in particular for optional rough or fine machining of the workpiece. The mutually independent displaceability of the two tool spindles is beneficial to the desired very narrow yoke construction, because the tool spindle not required at any one time may be moved upwards out of collision range. The narrower the yoke, the more rigidly it may be constructed, to allow high machining precision. However, this construction also allows simple cross-grinding adjustment without any additional adjusting means via the available CNC movement axis of the horizontal slide.
A workpiece loading means may be provided, which is vertically displaceable together with the tool spindle and shares in the CNC-controlled movements of the relevant tool spindle and therefore advantageously does not require any additional CNC axes.
The workpiece holding means can comprise two workpiece spindles with the axes of rotation lying in a plane with the swivel axis of the yoke. Of particular advantage is the position of the axes of rotation of the two tool spindles relative to the swivel axis of the yoke, because the centre of the workpiece thereby substantially remains in the area of the swivel axis. The very short lever ratios thereby allowed offer considerable advantages for the precision of angular positioning.
To reduce device idle times a tool changer may be arranged between the side walls of the frame, which tool changer may be displaced parallel to the gantry and with which a replacement tool may be brought very rapidly into the machining area of the device.
Given the aim of achieving as high as possible a degree of machining precision, it is advantageous for the frame to consist of polymer concrete, since this allows better vibration damping, less influencing of the temperature and active temperature control of the machine frame.